Do rooftop temperatures accurately represent the air temperatures that are experienced at ground level? The importance of weather station instrument exposure for measurement accuracy and consistency is well known. In order that these measurements be both spatially and temporally comparable, adherence to exposure standards is crucial. The standard exposure for National Weather Service (NWS) instruments is 1.5 meters above a ground surface representative of the surrounding environment. Recently the NWS has made an effort to replace roof stations within their station network. At the same time, there has been a rapid growth of private-sector weather station networks. Many of these stations are roof-mounted systems, thus raising the concern that roof-caused biases are present, particularly with air temperature measurements.

Previous work studying rooftop-ground temperature differences has largely been limited to case studies at individual rooftop sites. The measured differences from these various cases exhibit considerable variability, making it very difficult to detect any existing overall patterns in the relationships between rooftop and ground temperatures. To help address this problem, daily maximum and minimum temperatures are obtained from both the Washington, DC and Denver, CO metropolitan areas for the period 1999-2000. Descriptive statistics are first computed for the entire dataset and then separately for both clear and cloudy conditions. Overall sample averages indicate that rooftop sites have warmer minimum temperatures than ground sites, a trend observed in all seasons. For maximum temperatures, the results are less conclusive, although rooftop sites are still warmer than ground sites. Contour maps of rooftop-ground temperature differences are analyzed. These maps are useful both for locating stations that have a large rooftop-ground temperature difference and for better understanding the variability of these temperature differences. Current individual point studies focus on the periods of highest solar zenith angle in June and July.